1. Field of the Invention
This invention relates to a ferroelectric memory using ferroelectric capacitors.
2. Description of the Related Art
Recently, a ferroelectric capacitor has been employed as a memory to hold two valued charges. As for a memory of this type, a memory cell has been disclosed, for instance, by Unexamined Japanese Patent Application Hei-2-304796/(1991) FIG. 5 shows an electrical equivalent circuit of the memory cell, and FIG. 6 shows the structure of the same.
The memory cell, as shown in FIG. 5, comprises: a switching element, namely, a field-effect transistor 110, and a signal charge storing capacitor 120 using a ferroelectric substance. The field-effect transistor 110 has a gate electrode 111, a drain electrode 112, and a source electrode 113. The gate electrode 111 is connected to a word line WL, and the drain electrode 112 is connected to a bit line BL. The capacitor 120 comprises: a ferroelectric film 123; and two electrodes 121 and 122 formed on both sides of the ferroelectric film 123, respectively. The electrode 121 is connected to the source electrode 113 of the field-effect transistor 110, and the electrode 122 is connected to a ground line Vss or to a drive line DL. The ferroelectric film 123 is, in general, made of lead titanium zirconate (called "PZT").
The structure of the memory cell thus organized will be described with reference to FIG. 6 in brief.
A field oxide film 102 is formed by selective oxidation of the surface of a silicon substrate 101, thus defining a element forming region In the region, the field-effect transistor 110 is formed which consists of a gate electrode 111 covered by an oxide film 103, a drain region 112a, and a source region 113a. The lower electrode 121, the ferroelectric film 123, and the upper electrode 122 are formed on the source region 112a in the stated order, to form the capacitor 120. A metal conductor 104 is formed, as the bit line BL, on the drain region 112a, and a metal conductor 105 is formed, as the ground line Vss or the drive line DL, on the upper electrode 122.
The storage of charge of the ferroelectric capacitor in the above-described conventional non-volatile memory will be described with reference to FIGS. 7 and 8. FIG. 7 is an explanatory diagram showing the conventional capacitor formed on a semiconductor substrate. In FIG. 7, reference characters a and b designate the terminals of the capacitor. When voltage is applied across those terminals a and b of the capacitor, an amount of charge stored in the ferroelectric film 123 between the electrodes 121 and 122 is as shown in FIG. 8, in which the horizontal axis represents field strengths E and the vertical axis, amounts of polarization P. As the voltage between the terminals a and b changes, the amount of polarization of the ferroelectric film 123 changes as 0.fwdarw.A.fwdarw.B.fwdarw.C.fwdarw.D.fwdarw.E.fwdarw.F.fwdarw.G.fwdarw.B, thus showing a hysteresis characteristic.
When the field strength between the electrodes 121 and 122, after being raised to E.sub.sat much larger than E.sub.0, is returned to 0, then an amount of polarization P.sub.s (called "spontaneous polarization") remains in the ferroelectric film 123. Similarly, when the field strength between the electrodes 121 and 122, after being decreased to -E.sub.sat, is returned to 0, then an amount of polarization -P.sub.s remains in the ferroelectric film 123. With those positive and negative spontaneous polarizations corresponding to data "1" and "0" write states, the capacitor 120 provides a read signal charge Q represented by the following equation: EQU Q=2P.sub.s .multidot.S (Coulomb)
where S is the capacitor's area.
The spontaneous polarization P.sub.s is determined from the composition and thickness of the ferroelectric film 23.
In a conventional ferroelectric memory of this type, one ferroelectric capacitor holds binary charges. Hence, in order to provide multi-valued charges, it is necessary to provide as many ferroelectric capacitors as the number of charges required.